Instrumentation on avalanche photo diode gain stabilisation circuit - uses comparison between normal unity gain photocurrent and avalanche current to regulate reverse bias voltage

摘要

The circuit provides gain stabilisation of an instrumentation avalanche photo diode to be used for attenuation and band pass measurements in optical fibres. Gain stabilisation is provided by varying the applied bias voltage as a function of an error voltage derived by comparing the avalanche photo current to the primary photocurrent in the ratio of the desired avalanche gain. The primary photocurrent is measured and stored at unity gain by temporarily lowering the reverse bias voltage on the photo diode or by using an auxiliary photo diode under the same conditions illuminated by a fraction of the incident flux. The diode (10) is connected to a variable bias supply (30) controlled by a feedback loop and to a current amplifier with the desired gain (23). A two input comparator (26) is connected to the photo diode and the amplifier output and its output in the error signal which controls the bias voltage. An auxiliary supply (31) is connected to the diode by a switch (32) and the photocurrents connected by switches (21,29) to two storage condensers (22,25) storing voltages proportional to currents. Sequential control of the field effect transistor switches is effected by a time base circuit.